The primary goals of this research are to investigate nutrient allocation and costs during reproduction in the common marmoset and the cotton-top tamarin. Specifically to be examined is whether manipulating the energy or protein content of the diet effects ovulation number, gestation or lactation. This research will improve our understanding of the nutritional needs of these species which are important to biomedical research. Perhaps more importantly, this study will assess the potential of the common marmoset as a model for the study of nutritional effects on maternal and infant health. Reproductive function and body condition will be examined in animals exposed to one of the following dietary regimes: Period 1 Period 2 Period 3 1. Moderate Energy/ High Energy/ High Energy/ Moderate Protein High Protein Low Protein 2. Moderate Energy/ Moderate Energy/ Moderate Energy/ Moderate Protein High Protein Low Protein Subjects will remain on one diet through two pregnancies/lactation periods, during which the following parameters will be measured: (a) ovulation number; (b) fetal growth and mortality; (c) neonatal growth and mortality; (d) maternal body composition; (e) infant body composition; (f) milk yield and composition; (g) maternal behavior. A hierarchial linear model will be used to assess the inter-relation between these variables and diet. On the higher energy diets, it is expected that ovulation will increase and fetal mortality will decrease. Postpartum, increased energy intake may reduce or eliminate compromise of maternal body condition, while increasing milk yield, evidence regarding probably changes in milk composition is equivocal. The increased litter size expected may reduce the extent to which postnatal energy increases enhance growth and survival (i.e. these effects may be litter-size dependent). Increases in dietary protein may increase fetal growth rates and increase milk yield, resulting in higher infant growth rates as well; however, it is possible that decreased fetal survival will be seen at the highest protein levels. After elucidating the effects of these diet changes, studies may be designed in which the effects of specific restrictions (under nutrition) are assessed. Finally, the role of metabolic hormones (particularly insulin) as mediators of ovulation number will be examined, if time and subject availability allows.